Orthogonal Frequency-Division Multiple Access (OFDMA) is a multi-user version of the Orthogonal Frequency-Division Multiplexing (OFDM) digital modulation technology. In wireless OFDMA systems, however, multipath is an undesirable common propagation phenomenon that results in radio signals reaching the receiving antenna by two or more paths. Signal variations in amplitude or phase resulted from multipath are also referred as channel response. Transmission techniques, in which a transmitter makes use of the channel response between the transmitter and a receiver, are called close-loop transmission techniques. In MIMO applications, close-loop transmission techniques are much more robust as compared with open-loop MIMO techniques.
One method of providing channel information to the transmitter is via the use of an uplink (UL) sounding channel. Channel sounding is a signaling mechanism where a mobile station transmits sounding signals on an uplink channel (from the mobile station to the base station) to enable a base station to estimate the UL channel response. Channel sounding assumes the reciprocity of uplink and downlink channels, which is generally true in Time Division Duplexing (TDD) systems. Because the frequency bandwidth of the UL transmission encompasses the frequency bandwidth of the DL transmission in TDD systems, UL channel sounding can enable DL close-loop transmissions. UL channel sounding can also enable UL close-loop transmissions in both TDD and FDD systems. For example, the base station can choose the best precoding weights (vectors/matrices) to be used for data transmission from the mobile station.
In wireless OFDMA systems, a resource block is defined as a two-dimensional radio resource region comprising a number of consecutive sub-carriers (also referred as frequency tones) by a number of consecutive OFDM symbols (also referred as time slots). A resource block is the smallest unit for radio resource partition. For both DL and UL transmissions, the IEEE 802.16m specification defines 5-symbol resource block as 18 sub-carriers by five OFDM symbols, 6-symbol resource block as 18 sub-carriers by six OFDM symbols, and 7-symbol resource block as 18 sub-carriers by seven OFDM symbols. For UL transmission, the IEEE 802.16m specification additionally defines 5-symbol resource block as 6 sub-carriers by five OFDM symbols, 6-symbol resource block as 6 sub-carriers by six OFDM symbols, and 7-symbol resource block as 6 sub-carriers by seven OFDM symbols. The 6-subcarrier resource blocks are further called as a resource tile. Under a fixed number of OFDM symbols, an 18-subcarrier resource block accommodates three tiles. Moreover, the 5-symbol, 6-symbol, and 7-symbol resource blocks are called as type-3, type-1, and type-2 resource blocks, respectively. The IEEE 802.16m specification also defines corresponding pilot patterns for various predefined resource block sizes.
To ensure channel estimation quality of data traffic channel, sounding signals transmitted via an UL sounding channel are not allowed to collide with the original pilots that are allocated in various predefined UL resource blocks. Thus, in wireless OFDMA systems, it is desirable to design and allocate sounding channels based on predefined resource block sizes.